X-rays from Earth

Later this year, astronomers plan to use NASA's powerful
Chandra x-ray telescope to look at something new: Earth

August
18, 2003: Years ago when astronomers were first planning
NASA's Chandra X-ray Observatory, they didn't have planets in
mind. Planets were too peaceful for x-ray astronomy. Chandra
was going to reveal colliding galaxies, sizzling-hot whirlpools
around black holes, exploding stars--in other words, violent
sources of high-energy x-rays. Not planets.

"But planets have turned out to be pretty interesting
x-ray sources," says astronomer Ron Elsner of the Marshall
Space Flight Center. Jupiter, for instance, has a pulsating hot
spot near its north pole. Mars and Venus sparkle like disco balls.
Even the moon emits x-rays.

Right: A Chandra X-ray Observatory image of
the planet Venus. The night side of the planet is on the left,
the sparkling dayside on the right. [more]

And Earth? Although a few x-ray satellites have looked at
Earth before, the world we live on remains mostly terra incognita--a
mystery. Soon this will change. "Later this year we're going
to point Chandra at Earth," says Elsner. It will be the
first time astronomers have carefully studied our own planet
using the powerful x-ray telescope.

Elsner isn't sure what Chandra will see. "Earth might
look like Mars or Venus," he says. Those planets slowly
sparkle because they are bathed in x-rays from the sun. When
an x-ray photon hits the atmosphere of Mars or Venus, it is absorbed
by an atom--then re-emitted. Every few seconds there's a tiny
flash of x-radiation caused by this process, called "fluorescence."
The moon sparkles, too, although it has no atmosphere. There
the fluorescence takes place on the ground.

On the other hand, Earth might look like Jupiter. "Jupiter
sparkles in the same way that Venus and Mars do," notes
Elsner, "but Jupiter has something extra: an auroral x-ray
hot spot."

Jupiter's
hot spot is caused by heavy ions such as O6+ crashing
down onto the polar atmosphere. O6+ is an oxygen atom
missing 6 of its 8 electrons, and it wants them back. When these
strongly charged ions enter Jupiter's atmosphere they steal electrons
from nearby molecules. Chemists call this a "charge exchange
reaction." It's done with so much energy near Jupiter's
north pole that the reaction emits x-rays.

Above: Every 45 minutes an x-ray source blinks near
Jupiter's north magnetic pole. This animation, based on data
from the Chandra X-ray Observatory, shows the hot spot pulsing
15 times during one complete 10-hour rotation of the giant planet.
[more]

Where do these ions come from? And why does the hot spot sometimes
pulse--on and off every 45 minutes? "These are mysteries,"
says Randy Gladstone of the Southwest Research Institute, who
works with Elsner observing planetary x-rays.

"One possibility is that the oxygen ions might come from
the solar wind," he speculates. It's not easy to strip 6
electrons from oxygen, but it happens all the time in the million-degree
solar corona. The solar wind, which starts in the corona, blows
such ions throughout the solar system.

Like Jupiter, Earth has a strong magnetic field that can funnel
solar wind ions to the poles. Perhaps, like Jupiter, our planet
has a flashing hot spot. The possibility is what prompted Gladstone
in 2001 to propose the upcoming Chandra observations.

"This is a technically
challenging observation," says Gladstone. Chandra is a space
telescope traveling around Earth in an elliptical orbit. It comes
as close to Earth as 10,000 km and ranges as far away as 140,000
km--a third of the distance to the moon. Because of this elongated
orbit, "Earth is not a stationary target."

It's moving and big. The planet completely fills Chandra's
field of view. "When Chandra is pointing at Earth, it can't
see anything else--including the stars we normally use for guidance,"
notes Elsner. "We have to guide the telescope using nothing
but gyros."

Would it work?

"We had to find out," says Elsner. So he and colleagues
tried pointing Chandra at Earth earlier this year. It was just
a glimpse, but enough to prove "that we could successfully
track Earth and gather data."

All that remains is the real thing: a long look at our planet.
"We've been granted 12,000 seconds (3.3 hours) of observing
time," says Elsner. But they won't use it all at once. What
if Earth's appearance changes from day to day, or week to week?
A single snapshot could be misleading. "We plan to divide
our time into ten 1,200-second pieces and observe Earth on ten
separate days."

Chandra can't see the whole Earth at once.
"Our field of view is limited to an area 1100 km wide,"
says Gladstone. For comparison, the radius of Earth is 6378 km.
"We've decided to examine the polar regions first. If Earth
has a Jupiter-like beacon, that's where it will be."